Heap leaching

ABSTRACT

A heap of a material to be leached to recover a valuable metal from the material includes an electromagnetic heating system to generate heat in situ in the heap.

TECHNICAL FIELD

The present invention relates to leaching a material containing avaluable metal.

The present invention relates particularly, although by no meansexclusively, to leaching a material in the form of a sulphidic orecontaining a valuable metal.

The present invention relates particularly, although by no meansexclusively, to leaching a sulphidic copper-containing ore that includescopper-containing minerals, such as chalcopyrite.

BACKGROUND ART

In conventional heap and dump leaching of copper sulphide containingminerals, mined ore is stacked into heaps, aerated through directinjection of air via aeration pipes extending into the heap and/or bynatural convection through exposed sides of the heap, irrigated with anacid solution for extraction of copper into solution, and the copper issubsequently recovered from solution by a range of recovery optionsincluding solvent extraction and electrowinning (SX/EW), cementationonto more active metals such as iron, hydrogen reduction, and directelectrowinning. Leaching may be enhanced by the use of microorganisms,such as acidophilic bacteria that grow on the surface and in the cracksof ore fragments in a heap.

Generally, heap and dump leaching (hereinafter referred to as “heapleaching”) provides lower metal recoveries than other metallurgicalprocess options for recovering copper from copper-containing ores, suchas milling and flotation that produces copper-containing concentratesthat are then smelted to produce copper metal. Consequently, heapleaching tends to be reserved for low grade ore types (typically, 0.5-2wt. %) that have at least a proportion of readily recovered copper, butwhere crushing/milling costs per unit of copper are too high to supporta concentrator approach, or where mineral liberation and othercharacteristics (e.g. arsenic content) will not support production ofdirectly useable or saleable concentrates.

The above description is not to be taken as an admission of the commongeneral knowledge in Australia or elsewhere.

SUMMARY OF THE DISCLOSURE

The applicant, through a group company and research partners, hascarried out research and development work on heap leachingcopper-containing ores and has made a number of findings in the courseof the heap leaching work and in the course of work on other technologyprojects. The present invention is an outcome of those findings.

Temperature control in a heap is an important issue from the viewpointof having leaching reactions occur at an acceptable rate. Temperaturecontrol may be an issue at start-up and during the course of a heapleaching operation. The temperature requirements may vary during thecourse of a heap leaching operation due to reactions within the heap andexternal conditions such as external temperature. By way of example, oneof the challenges of a heap leaching operation, particularly when thegrade is low and the climate is cold, is getting the internaltemperature of a heap to a point where reactions are self-sustaining atan acceptable rate. Moreover, temperature control at scale within a heapis difficult, especially in cold conditions and often results inconventional heap leaching operations being limited to achieving highextraction rates.

The present invention is based on a realisation made during the courseof the research and development work that electromagnetic radiation,particularly radio frequency radiation, can selectively heat a leachsolution and/or sulphide mineralisation (depending on mineralogy) commonto copper bearing ores and not heat the host rock, i.e. gangue, and isuseful on this basis.

In general terms the present invention provides a heap of a material tobe leached to recover a valuable metal from the material, the heapincluding an electromagnetic heating system to generate heat in situ inthe heap.

The term “electromagnetic heating system” is understood herein to meanany system based on the use of an electromagnetic field to generate heatdirectly or indirectly in a heap of a material.

The present invention may provide a heap of a material to be leached torecover a valuable metal from the material, the heap including anelectromagnetic heating system in the form of a system for exposing theheap to electromagnetic radiation to generate heat in situ in the heap.

The system for exposing the heap to electromagnetic radiation may beoperable to selectively heat minerals containing valuable metal comparedto non-valuable gangue material in the heap.

The system for exposing the heap to electromagnetic radiation may beoperable to heat minerals containing valuable metal to a uniformtemperature range throughout at least a substantial part, typically atleast 90% of the heap.

The system for exposing the heap to electromagnetic radiation may beoperable to selectively heat leach liquor compared to non-valuablegangue material in the heap. The research and development work foundthat selectively heating leach liquor compared to non-valuable ganguematerial is an effective way of heating the whole heap to a uniformtemperature range. Typically, leach liquor is well distributed through aheap and there is heat transfer via conduction from the liquor to thesurrounding heap material.

The system for exposing the heap to electromagnetic radiation may beoperable to heat leach liquor to a uniform temperature range throughoutat least a substantial part, typically at least 90% of the heap.

The system for exposing the heap to electromagnetic radiation may beoperable to heat the heap liquor to at least 50° C., preferably in therange between 45° C. and 65° C., and typically about 55° C. when thematerial in the heap includes sulphidic copper-containing ore withchalcopyrite as a copper-containing mineral in the ore.

The system for exposing the heap to electromagnetic radiation may beoperable to heat the heap liquor to less than 85° C. when the materialin the heap includes sulphidic copper-containing ore with chalcopyriteas a copper-containing mineral in the ore.

In situations where the material includes sulphidic ore containing avaluable metal and a leaching operation on the heap uses acidic leachliquor, the electromagnetic radiation system heats the metal sulphidesand the leach liquor and generates heat to initiate and maintainreactions in the heap.

The electromagnetic radiation may be any suitable radiation.

The electromagnetic radiation may be radio frequency radiation.

The electromagnetic radiation may be in a lower frequency end of theradio frequency radiation band of radiation.

The lower frequency end of the radio frequency radiation band ofradiation may be 1-100 MHz.

The radio frequency radiation may preferably be selected to be between5-45 MHz.

In situations in which the electromagnetic radiation is radio frequencyradiation, the system for exposing the heap to electromagnetic radiationmay include a radio frequency generator, an array of electrodes toradiate the radio frequency energy into the heap, and transmission linesinterconnecting the radio frequency generator and the electrodes.Reference in the specification to electrodes includes reference toantennae.

By way of example, the system for exposing the heap to electromagneticradiation may include a series of spaced-apart electrodes positioned inthe heap and an electrical source connected to the electrodes that isoperable to generate electromagnetic waves that have the frequency inthe radio frequency band.

The electrodes may be arranged to extend vertically into the heap.

The electrodes may be arranged to extend horizontally into the heap.

The electrodes may be in the form of an elongate central conductor andan outer coaxial conductor, with an annular gap between the conductors.

The electrodes may be in any suitable form and any suitable arrangement.The size and spacing and orientation of the electrodes may be selectedas required.

By way of example, each electrode may include an elongate centralconductor and an outer coaxial conductor, with an annular gap betweenthe conductors.

The heap may include an array of sleeves extending into the heap and theelectrodes may be replaceably positioned in the sleeves.

The sleeves may be made from material that is transparent to theelectromagnetic radiation.

The system for exposing the heap to electromagnetic radiation mayinclude a shield to confine the electromagnetic radiation within theheap.

The shield may be in the form of metal mesh on the outside of the heapthat acts as a Faraday cage that prevents electromagnetic radiationbeing transmitted outside the heap.

The heap may be any suitable size and shape. By way of example, the heapmay be elongate with a pair of parallel longer sides and a pair ofparallel shorter sides (which may be described as “ends”).

The material may be a sulphidic ore containing a valuable metal.

The sulphidic ore may be a sulphidic copper-containing ore that includescopper-containing minerals.

The copper-containing minerals may include chalcopyrite.

The heap may include an aeration system for supplying air to the heapunder natural convection or forced air flow conditions.

The heap may include a system for supplying leach liquor to the heap forleaching the valuable metal from the material in the heap.

The leach liquor may be an acidic solution, typically pH 1.5-2.

The acidic solution may include sulphuric acid.

The leach liquor may be pre-heated.

The heap may include a system for draining a pregnant leach solutionthat contains the valuable metal leached from the material in the heapfrom the heap.

In general terms, the present invention also provides a method of heapleaching a valuable metal from a material that includes the steps of (a)supplying a leach liquor to a heap of the material to leach the valuablemetal from the material and (b) controlling the temperature in the heapby electromagnetic heating that generates heat in situ in the heapduring the course of the method.

The present invention may provide a method of heap leaching a valuablemetal from a material that includes the steps of (a) supplying a leachliquor to a heap of the material to leach the valuable metal from thematerial and (b) controlling the temperature in the heap byelectromagnetic heating by exposing the heap to electromagneticradiation to generate heat in situ in the heap during the course of themethod.

The term “electromagnetic heating” is understood herein to mean using anelectromagnetic field to generate heat directly or indirectly in a heapof a material.

Step (b) may include controlling the temperature in the heap by exposingthe heap to electromagnetic radiation and selectively heating mineralscontaining valuable metal compared to non-valuable gangue material inthe heap.

Step (b) may include controlling the temperature in the heap by exposingthe heap to electromagnetic radiation and selectively heating leachliquor compared to non-valuable gangue material in the heap.

Step (b) may include controlling the temperature in the heap by exposingthe heap to electromagnetic radiation and selectively heating leachliquor to at least 50° C., preferably in the range between 45° C. and65° C., and typically about 55° C. when the material in the heapincludes sulphidic copper-containing ore with chalcopyrite as acopper-containing mineral in the ore.

Step (b) may include controlling the temperature in the heap by exposingthe heap to electromagnetic radiation and selectively heating leachliquor to less than 85° C. when the material in the heap includessulphidic copper-containing ore with chalcopyrite as a copper-containingmineral in the ore.

The method may include operating the electromagnetic radiation system asrequired so that the temperature of the heap is within a targettemperature range.

The method may include monitoring the heap temperature and exposing theheap to electromagnetic radiation as required having regard to themonitored temperature so that the temperature of the heap is within atarget temperature range.

The target temperature range may be a range that applies across thewhole heap.

Alternatively, the target temperature range may be different ranges indifferent sections of the heap. For example, there may be a highertarget temperature range in an outer section of the heap than in aninner section of the heap.

The method may include aerating the heap.

The method may include supplying leach liquor to the heap for leachingthe valuable metal from the material in the heap.

The method may include collecting a pregnant leach solution thatcontains the valuable metal leached from the material and recovering thevaluable metal from the pregnant leach solution.

The material may be a sulphidic ore containing a valuable metal.

The sulphidic ore may be a sulphidic copper-containing ore that includescopper-containing minerals, such as chalcopyrite.

In the case of sulphidic copper-containing ore that includecopper-containing minerals, step (b) may include controlling thetemperature in the heap to be at least 50° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described further with reference to theaccompanying drawings, of which:

FIG. 1 is a diagram that illustrates a heap of a material containing avaluable metal to be leached in accordance with the present invention ingeneral terms;

FIG. 2 is a graph of loss factor (ε″) against frequency at ISM bands fordifferent concentrations of standard sulphuric acid leach liquor used toleach copper-bearing ores;

FIG. 3 is a graph of loss factor (ε″) against frequency at ISM bands forsome common minerals found in copper-bearing ores;

FIG. 4 is a perspective view of one embodiment of a heap of a materialcontaining a valuable metal to be leached in accordance with the presentinvention;

FIG. 5 is a vertical cross-section through a section of the heap shownin FIG. 3 which illustrates the arrangement of electrodes in the heap inthat section of the heap in very diagrammatic form;

FIG. 6 is a diagram of part of one of the electrodes in the heap shownin FIGS. 4 and 5;

FIG. 7 is a side view of another embodiment of a heap of a materialcontaining a valuable metal to be leached in accordance with the presentinvention which includes an outer mesh that acts as a shield forconfining electromagnetic radiation within the heap;

FIG. 8 is a top plan view of another embodiment of a heap of a materialcontaining a valuable metal to be leached in accordance with the presentinvention; and

FIG. 9 is a cross-section along the line 9-9 in FIG. 8.

SPECIFIC DESCRIPTION OF EMBODIMENTS

The following description is in the context of heap leaching a sulphidiccopper-containing ore. It is emphasised that the present invention isnot confined to this type of ore and extends more generally to anymaterial that includes a valuable metal.

The recovery of copper using heap leaching systems is a proventechnology and has been used successfully used. The performance of thesesystems is primarily governed by the temperature within the heap, andthis has been reported in both laboratory and field studies. Thenumerous bacterial strains used in such systems have particulartemperature windows in which they proliferate and drive the extractionof soluble copper into leach solutions. If these temperature regions arenot reached then recovery of copper can be poor.

Controlling the temperature within heap leaching systems that rely onbacterial activity is essential, particularly in cold environmentalconditions, to allow acidophilic bacteria to grow. Yet the large numberof complex and dependant chemical steps, as well as the influence oftemperature altering process conditions such as air flow, irrigationrate, and evaporation makes control of the temperature difficult, if notimpossible, with the current state of the art.

The applicant has found that electromagnetic heating, as describedabove, makes it possible to selectively heat a leach solution and/orsulphide mineralisation common to copper-bearing ores over the hostrock, i.e. the gangue. As a consequence, the localised temperature ofthe environment around the acidophilic microbes attached to the surfacesulphide mineralisation can therefore, be optimised, to enhance both therate, and overall copper recovery.

In particular, the research and development work mentioned above foundthat the application of electromagnetic energy to sulphidiccopper-containing ores in a heap leach could produce near instantaneous,in situ, volumetric and phase selective heating of the leach solution inthe heap. The work found that selectively heating leach liquor comparedto non-valuable gangue material is an effective way of heating the wholeheap to a uniform temperature range. Typically, leach liquor is welldistributed through a heap and there is heat transfer via conductionfrom the liquor to the surrounding heap material.

The use of energy in the radio frequency range of the electromagneticradiation spectrum is preferred because the penetration depth isproportional to the wavelength of the applied electromagnetic energy andthis is of the order of tens if not hundreds of meters in the radiofrequency range. Therefore, the separation between radiating electrodescan also be of the same order, allowing economical design of aradio-frequency heating system. In addition, the heating selectivelyalso increases at longer wavelengths.

FIG. 2 is a graph of loss factor (ε″) against frequency at ISM bands fordifferent concentrations of standard sulphuric acid leach liquor used toleach copper-bearing ores. FIG. 3 is a graph of loss factor (ε″) againstfrequency at ISM bands for some common minerals found in copper-bearingores.

The graph of FIG. 3 shows greater separation between mineralisation ofinterest, such as chalcopyrite, and gangue at lower frequencies. Quartzis often the predominant mineral in common copper-bearing ores and isessentially transparent to radio frequency energy, given its very lowdielectric loss (ε″). Yet the primary copper mineral—chalcopyrite—has,relatively, a very dielectric high loss. So it exhibits a much greaterdegree of heating in the same radio frequency field. It then followsthat mineralogical composition of an ore fragment defines its heatingresponse. Those fragments high in chalcopyrite will heat better thanthose that have comparatively less chalcopyrite.

It can be seen from FIG. 2 that (a) the loss factor for each acidconcentration decreases as the frequency of the applied electromagneticenergy increases and (b) the loss factor at a given frequency increaseswith acid concentration of the leach liquor. It can be seen from FIG. 3that as the frequency of the applied electromagnetic energy decreases,the spread in measured loss factors of the selected minerals increases.It can also be seen from a comparison of FIGS. 2 and 3 that the lossfactors of the leach liquor across the concentration range at lowerfrequencies of the applied electromagnetic energy is significantlyhigher than the loss factors for the selected minerals, includingchalcopyrite which has a high loss factor. For example, at 20 MHz, theloss factor for a 10% sulphuric acid leach liquor is approximately 400and the loss factor for chalcopyrite is approximately 70. Given thatloss factor is essentially a measure of heating in response to anapplied field, it can be inferred from these results that at lowerfrequencies, typically less than 100 MHz, significant heating of leachliquor is possible and greater degrees of selective heating of mineralscan be achieved. The radio frequency radiation is preferably selected tobe between 5-45 MHz, more particularly with a wavelength in the regionof 25 metres.

It is the above-described selective heating of leach liquor andphase-selective heating of valuable minerals (such as chalcopyrite) andnon-valuable minerals (such as quartz) that underpins the mineralprocessing technology of the invention.

Given the large-scale of heap leaching operations, heating the bulkagglomerate of the ore heap to the required temperature for microbialactivation, would require huge energy input, and be prohibitivelyuneconomical. By delivering targeted radio frequency energy toselectively heat the leach solution and/or sulphide mineralisation (towhich the acidophilic microbes are attached), the optimum temperaturefor acidophilic microbial proliferation can be achieved without wastingenergy heating the bulk ore from which no benefit is derived in terms ofcopper recovery.

In a heap leaching system the mineral ore is subjected to a series ofcrushing and screening processes to prepare agglomerates or aconcentrate which is then placed on a leach pad. This leach pad includesan impermeable geotextile on which a series of air pipes and collectionlines are laid. Drip lines are then placed across the top of the orebed, through which an acidic leach solution is applied to the top of theore heap. The composition of the leach solution is typically arelatively dilute sulphuric acid solution at pH 1.5-2 and an oxidiser.But in the case of bioleaching, a bacterial inoculate is used in placeof a chemical oxidiser. As the solution passes through the agglomeratedore, metals are extracted into the solution as a soluble salt. These arethen stripped from the pregnant leach solution by solvent extraction.The metal of interest is when extracted the electrolyte solution byelectrowinning to produce a high purity cathode metal. The leachsolution is then recycled into the irrigation system.

The heaps shown in FIGS. 4, 5 and 7-9 are standard forms in terms of thebasic shape and size of the heaps and insofar as the heaps include minedore that has been processed, for example by being crushed and screened,in accordance with standard practice for forming heaps. Morespecifically, the present invention does not extend to the particularshape and size of the heap and does not extend to the physicalcharacteristics of the ore. In addition, more specifically, the presentinvention also extends to heaps of previously discarded material frommining operations.

The embodiment of the heap shown in FIGS. 4 and 5 is basically elongatewith a pair of parallel inclined longer sides and a pair of parallelinclined shorter sides (which may be described as “ends”) that extendaround the entire perimeter of the heap, and with a generally flat top.

With reference to the FIG. 4, the heap of the sulphidiccopper-containing ore includes:

(a) a system generally identified by the numeral 3 for exposing the heapto electromagnetic radiation in the form of radio frequency radiation toselectively generate heat in the heap;

(b) an air impermeable barrier in the form of a plurality ofprefabricated panels 5 positioned on the sides of the heap to excludeair flow through the sides;

(c) an aeration system to allow controlled air flow into the heap asrequired during the course of a heap leaching operation, the aerationsystem including a plurality of aeration pipes 11 that extendhorizontally through openings in the panels 5 into the heap, and theaeration system being arranged to supply air to the heap under naturalconvection or forced air flow conditions;

(d) a system generally identified by the numeral 15 for supplying aleach liquor to the top of the heap so that the leach liquor can flowdownwardly through the heap and leach copper from the ore; and

(e) a system (not shown) for discharging a pregnant leach solution thatcontains copper in solution that has been leached from the ore from theheap in a way that prevents flow of air into the heap via the dischargesystem.

The electromagnetic radiation system 3 makes it possible to control thetemperature of the leach liquor in the heap to a target temperaturerange substantially throughout the whole of the heap or to differenttarget temperature ranges in different sections of the heap. The system3 is suitable for use on large size heaps.

More particularly, when the electromagnetic radiation is selected to beradio frequency radiation toward the lower end of the radio frequencyband, such as 1-100 MHz, the ore penetration can be tens and up tohundreds of meters and thus provides an effective heating option forlarge heaps. Nevertheless, it is often preferable to have a minimumelectric field value which will decay exponentially away from theelectrode so the distance one electrode can heat in reality will be 10'sof meters and not hundreds of meters.

Moreover, on a microscopic scale, radio frequency radiation can heatcopper-containing minerals and the leach liquor very rapidly, whichprovides an opportunity for selective heating and leaching at thelocation of the valuable metal and, in particular, heating at the solidliquid interface. This is important in terms of leaching rate andrecovery.

The electromagnetic radiation system 3 shown in FIGS. 4 and 5 includes(i) a plurality of spaced-apart electrodes 13 arranged to extendvertically (or in any other suitable orientation) into the heap, (ii) anelectrical source (not shown) connected to the electrodes and operableto generate electromagnetic waves that have a frequency in the radiofrequency band, whereby in use radio frequency radiation is transmittedfrom the electrodes 13, (iii) a plurality of temperature sensors 17 formeasuring the temperature of the heap (and externally of the heap) inselected locations, and (iv) a controller (not shown) for receiving andprocessing the detected temperature data and controlling the operationof the electrical source to vary the operation of the electrical sourcein response to the detected temperatures as required to achieve a targettemperature range within the heap or different target temperature rangesin different sections of the heap.

FIG. 1 is a diagram that illustrates in general terms a heap of amaterial containing a valuable metal to be leached in accordance withthe method of the present invention. The heap shown in FIG. 1 is of thesame type as that shown in FIGS. 4 and 5.

FIG. 1 illustrates a heap 3 that includes a series of electrodes 13extending into the heap and a radio frequency generator 41 connected tothe electrodes 13 via transmission lines. The Figure also shows leachliquor, in this instance containing bacteria, being supplied to theheap. An important feature of the Figure, which is evident from thecolour coding of the heap and by reference to the temperature/colourscale on the left hand side of the Figure, is that the radio frequencyenergy transmitted from the electrodes 13 heats the heap to atemperature of 50° C. substantially through the heap, which facilitatesleaching rate and recovery. This heating of the whole heap 3 is a resultof electromagnetic energy producing near instantaneous, in situ,volumetric and phase selective heating of the leach solution in the heapand heat transfer via conduction from the liquor to the surrounding heapmaterial.

With reference to FIG. 6, each electrode 13 in the heap shown in FIGS. 4and 5 is in the form of an elongate central conductor 21 and an outercoaxial conductor 23, with an annular gap 25 between the conductors 21,23.

With further reference to FIG. 6, the electrode 13 is shown positionedin an outer sleeve 27, for example formed from a plastics material thatis transparent to radio frequency radiation. The electrode is removablypositioned in the sleeve 27. The outer sleeve 27 forms part of the heapshown in FIGS. 4 and 5. The outer sleeve 27 is a convenient housing forthe electrode 13.

In the embodiment shown in FIGS. 4 and 5, there is one electrode 13 ineach sleeve 27. In other embodiments of the invention, there may bedeliberately more sleeves 27 than electrodes 13, and the electrodes 13may be moved singly or in groups to selectively heat different sectionsof the heap. The electrodes are spaced to be between 4 and 8 meters fromeach other, and up to about a wavelength distance of the radiofrequency.

It is also noted that the electromagnetic radiation system 3 may beoperable to heat at different heating rates to achieve a given targettemperature range in a section of the heap during the course of a heapleach operation. For example, it may be desirable to operate at a higherheating rate during a start-up phase of a heap leach operation than at alater stage in the operation.

It is also noted that the electromagnetic radiation system 3 may beresponsive to other inputs in addition to heap temperature, such aschanges in the pH and flow rate of leach liquor.

It is also noted that the arrangement, including the structure and thespacing of the electrodes 11 may readily be determined as required oncethe mineralogy and other characteristics of the materials in the heapand the pH and other characteristics of the leach liquor (such as flowrate) and the operational requirements for a given heap are established.

The embodiment of the heap shown in FIG. 7 includes the same features asthe heap shown in FIGS. 4 and 5.

The additional feature of the FIG. 7 heap is a shield in the form ofsteel mesh 25 arranged on the outside of the heap that forms a Faradaycage to prevent transmission of radio frequency radiation from the heap.

The embodiment of the heap shown in FIGS. 8 and 9 includes the samebasic features as the heap shown in FIGS. 4 and 5.

The difference between the two heaps is that the electrodes 13 in theFIGS. 8 and 9 heap are positioned to extend horizontally into the heapfrom one side of the heap and are arranged at spaced intervals along thelength of the heap, with each electrode 13 being connected to a separateradio frequency source 29 via transmission lines 31.

A heap leach method for each of the above-described embodiments of heapsincludes the steps of (a) supplying a leach liquor to the heap to leachcopper from the sulphidic copper-containing ore in the heap and (b)controlling the temperature in the heap by electromagnetic heating byexposing the heap to electromagnetic radiation to generate heat in situin the heap during the course of the method. In any given situation, theselection of the required heating for the heap (including a target heaptemperature or range of temperatures for the heap) during the course ofa day and across a leach program will take into account a number offactors such as, for example, the leach liquor, the liquor flow rate,the characteristics of the material in the heap, such as the copperconcentration and mineralisation of the copper, the bacteria used in theheap, and external temperatures. The method also includes monitoring theheap, including monitoring temperatures within and externally of theheap and controlling heating as required to maintain a target heaptemperature or range of temperatures.

The advantages of the present invention include:

(a) direct selective heating of copper-containing minerals and leachliquor within a heap, without heating non-valuable gangue material inthe heap;

(b) an opportunity for enhanced leaching operations through heating atthe leach liquor and fragment interface;

(c) heating is easy to control via RF generator power input;

(d) very homogenous temperature throughout the whole heap possible;

(e) readily scaled-up to suit any sized heap;

(f) an opportunity for enhanced leaching operations through heating atthe leach liquor and fragment interface;

(g) radio frequency generators and transmission cables are proven robustprocess technology components;

(h) an opportunity for leaching at higher rates, including during lowexternal temperature periods;

(i) an opportunity to provide selective heating in a heap that may causeoxidation which may allow acceleration of reaction rates;

(j) heating into the structure of the ore fragments that may have abeneficial effect on creating fractures that facilitate leach liquorpenetration and/or improving overall reaction times;

(k) an opportunity to operate with larger fragments and save comminutioncosts because the impact of electromagnetic radiation is greater forlarger sized fragments;

(l) an opportunity to operate heaps independently of externaltemperature conditions;

(m) operating applicable for use on large heaps; and

(k) confinement of electromagnetic radiation within a heap is astraightforward and inexpensive option.

It has been found that the initial heap temperature during start-up hasa significant impact on the long term reaction rates in the heap. Theelectromagnetic heating system of the present invention is suitable forheating the heap during start-up to increase the initial and overallextraction rates.

Many modifications may be made to the embodiments of the presentinvention described above without departing from the spirit and scope ofthe invention.

By way of example, whilst the embodiments of the invention are describedin the context of heap leaching a sulphidic copper-containing ore, thepresent invention is not confined to this type of ore and extends moregenerally to any material that includes a valuable metal.

By way of further example, whilst the embodiments of the invention aredescribed in the context of an electromagnetic heating system that isbased on exposing a heap to radio frequency radiation, the invention isnot so limited and extends to exposing a heap to other bands of theelectromagnetic radiation spectrum.

By way of further example, whilst the embodiments of the invention aredescribed in the context of an electromagnetic heating system that isbased on exposing a heap to electromagnetic radiation, particularlyradio frequency radiation, the invention is not so limited and extendsto other types of electromagnetic heating system that are based on theuse of an electromagnetic field to generate heat directly or indirectlyin a heap of a material.

By way of further example, whilst the embodiments of the inventioninclude a particular from of the electrodes and particular arrangementsof the electrodes in heaps, the invention is not so limited and extendsto any suitable electrode configurations and arrangements of electrodes.

1. A heap of a material to be leached to recover a valuable metal from the material, the heap including an electromagnetic heating system to generate heat in situ in the heap.
 2. A heap of a material to be leached to recover a valuable metal from the material, the heap including an electromagnetic heating system in the form of a system for exposing the heap to electromagnetic radiation to generate heat in situ in the heap.
 3. The heap defined in claim 2 wherein the system for exposing the heap to electromagnetic radiation is operable to selectively heat leach liquor in the heap.
 4. The heap defined in claim 2 wherein the system for exposing the heap to electromagnetic radiation is operable to heat heap liquor to at least 50° C., preferably in the range between 45° C. and 65° C., and typically about 55° C. when the material in the heap includes sulphidic copper-containing ore with chalcopyrite as a copper-containing mineral in the ore.
 5. The heap defined in claim 2 wherein the system for exposing the heap to electromagnetic radiation is operable to heat heap liquor to less than 85° C. when the material in the heap includes sulphidic copper-containing ore with chalcopyrite as a copper-containing mineral in the ore.
 6. The heap defined in claim 1 wherein the electromagnetic heating system is operable to heat leach liquor and minerals containing valuable metal to a uniform temperature range throughout at least 90% of the heap.
 7. The heap defined in claim 1 wherein the electromagnetic radiation is radio frequency radiation.
 8. The heap defined in claim 7 wherein the electromagnetic radiation is in a lower frequency end of the radio frequency radiation band of radiation.
 9. The heap defined in claim 8 wherein the lower frequency end of the radio frequency radiation band of radiation is 5-45 MHz.
 10. The heap defined in claim 7 wherein in situations in which the electromagnetic radiation is radio frequency radiation, the system for exposing the heap to electromagnetic radiation includes a series of spaced-apart electrodes positioned in the heap and an electrical source connected to the electrodes that is operable to generate currents that oscillate at radio frequencies.
 11. The heap defined in claim 10 wherein the electrodes are arranged to extend vertically into the heap.
 12. The heap defined in claim 1 wherein the electromagnetic heating system includes a shield to confine the electromagnetic radiation within the heap.
 13. The heap defined in claim 12 wherein the shield is in the form of metal mesh on the outside of the heap that acts as a Faraday cage that prevents electromagnetic radiation being transmitted outside the heap.
 14. The heap defined in claim 1 or claim 2 wherein the material is a sulphidic ore containing a valuable metal.
 15. (canceled)
 16. A method of heap leaching a valuable metal from a material that includes the steps of (a) supplying a leach liquor to a heap of the material to leach the valuable metal from the material and (b) controlling the temperature in the heap by electromagnetic heating by exposing the heap to electromagnetic radiation to generate heat in situ in the heap during the course of the method.
 17. The method defined in claim 16 wherein step (b) includes controlling the temperature in the heap by exposing the heap to electromagnetic radiation and selectively heating minerals containing valuable metal compared to non-valuable gangue material in the heap.
 18. The method defined in claim 16 wherein step (b) includes controlling the temperature in the heap by exposing the heap to electromagnetic radiation and selectively heating leach liquor in the heap.
 19. The method defined in claim 16 wherein step (b) includes controlling the temperature in the heap by exposing the heap to electromagnetic radiation and selectively heating leach liquor to at least 50° C., preferably in the range between 45° C. and 65° C., and typically about 55° C. when the material in the heap includes sulphidic copper-containing ore with chalcopyrite as a copper-containing mineral in the ore.
 20. The method defined in claim 16 wherein step (b) includes controlling the temperature in the heap by exposing the heap to electromagnetic radiation and selectively heating leach liquor to less than 85° C. when the material in the heap includes sulphidic copper-containing ore with chalcopyrite as a copper-containing mineral in the ore.
 21. The method defined in claim 16 includes monitoring the heap temperature and exposing the heap to electromagnetic radiation as required having regard to the monitored temperature so that the temperature of the heap is within a target temperature range.
 22. The heap defined in claim 2 wherein the electromagnetic heating system is operable to heat leach liquor and minerals containing valuable metal to a uniform temperature range throughout at least 90% of the heap.
 23. The heap defined in claim 2 wherein the electromagnetic radiation is radio frequency radiation.
 24. The heap defined in claim 23 wherein the electromagnetic radiation is in a lower frequency end of the radio frequency radiation band of radiation.
 25. The heap defined in claim 24 wherein the lower frequency end of the radio frequency radiation band of radiation is 5-45 MHz.
 26. The heap defined in claim 23 wherein in situations in which the electromagnetic radiation is radio frequency radiation, the system for exposing the heap to electromagnetic radiation includes a series of spaced-apart electrodes positioned in the heap and an electrical source connected to the electrodes that is operable to generate currents that oscillate at radio frequencies.
 27. The heap defined in claim 26 wherein the electrodes are arranged to extend vertically into the heap.
 28. The heap defined in claim 2 wherein the electromagnetic heating system includes a shield to confine the electromagnetic radiation within the heap.
 29. The heap defined in claim 28 wherein the shield is in the form of metal mesh on the outside of the heap that acts as a Faraday cage that prevents electromagnetic radiation being transmitted outside the heap.
 30. The heap defined in claim 2 wherein the material is a sulphidic ore containing a valuable metal. 